Developments in silicon-based integrated circuit technology, including field-effect transistor (FET) technology, have provided greater device speed, increased integration density, and/or improved functionality. However, as transistor dimensions continue to scale-down, a variety of operational and structural problems may arise. For example, as the channel length of a transistor is reduced, short-channel effects such as punch-through, drain induced barrier lowering (DIBL), and increased leakage current may occur.
As such, alternative transistor designs are being developed to address problems associated with shrinking device dimensions while improving transistor performance. One alternative design involves the use of strained silicon in the channel region of the transistor. Strain may be created in crystalline silicon by applying layers of other materials to physically elongate or compress bonds between the crystal's atoms. For example, germanium atoms may replace some of the silicon atoms near a surface of a silicon wafer, and a thin layer of silicon may be grown on top of this silicon-germanium (SiGe) layer. Because germanium atoms are larger than silicon atoms, the distance between the atoms in the silicon-germanium lattice is greater than it is in pure silicon. As such, when a silicon layer is grown on top of a silicon germanium layer, the silicon atoms may line-up with the silicon-germanium lattice below, which may increase the distance between silicon atoms and thereby create strain in the silicon layer. This strain may enable electrical charges to pass more easily through the silicon lattice. Thus, carrier mobility may be increased in a transistor having a strained silicon channel region.
A particular application employing strained silicon, developed by Intel Corporation, is described by T. Ghani et al. in “A 90 nm High Volume Manufacturing Logic Technology Featuring Novel 45 nm Gate Length Strained Silicon CMOS Transistors” (IEDM 2003). As stated in Ghani et al., a PMOS transistor structure features an epitaxially grown strained silicon-germanium (SiGe) film embedded in the source/drain regions of a transistor by using a selective epitaxial growth process. A combination of compressive SiGe strain and embedded SiGe source/drain geometry induces a large uniaxial compressive strain in the channel region of the transistor.
Another application employing strained silicon, developed by IBM, is discussed by K. Rim et al. in “Fabrication and Mobility Characteristics of Ultra-thin Strained Si Directly on Insulator (SSDOI) MOSFETs” (IEDM 2003). As stated in Rim et al., a strained silicon (Si) directly-on-insulator structure is fabricated by a layer transfer technique. A thin layer of strained Si is epitaxially grown on a relaxed silicon germanium (SiGe) graded buffer. An insulator layer, which is to become the buried oxide layer, is formed on top of the strained Si layer by a combination of thermal oxidation and CVD oxide deposition. After planarizing the oxide surface with a chemical mechanical polishing step, hydrogen is implanted through the oxide and into the SiGe layer, and the wafer is bonded to a silicon wafer (“handle substrate”). Thermal annealing induces cavity formation within the SiGe buffer by the implanted hydrogen, and the bonded stack is split at the interface created by these cavities, leaving the buried oxide layer, the strained Si layer, and a layer of relaxed SiGe on the new handle wafer. After a thermal anneal step to strengthen the bonding interface, the SiGe layer on top of the strained Si layer is selectively removed, leaving only the strained Si layer on the buried oxide.